Graph is not remotely self-explanatory. Does "read time" mean how a reader spends on a page, or how long it takes to read the entire page? Does "social activity" refer to the reader or the article? What the fuck are the units? There's not a single number on the worthless thing.
You have to actually read the article to find out what they're really saying: people are more likely to share articles that they spent less time reading (note, not the same as "shorter articles"), and vice versa.
...Maybe, because right before they say (imply?) that, they say "We looked at 10,000 socially-shared articles and found that there is no relationship whatsoever between the amount a piece of content is shared and the amount of attention an average reader will give that content." I dunno, because (drum roll) I didn't read the rest of the article. (rimshot)